1. Field of Invention
The field of the present invention relates to the inspection of manufactured parts. Specifically, firstly it relates to the inspection of finished parts for dimensional accuracy. Secondly, it relates to inspecting the host machine itself for positional accuracy.
2. Prior Art
Milling machines, jig borers, and similar machinery are used to work a piece of raw material into a part of a precisely specified geometrical shape. Such machinery usually has a rotating spindle and a way of securely holding a tool, such as a drill bit or an end mill, co-axial with said spindle. The work piece is securely held on a precision XYZ stage of such a machine and advanced a known distance against the cutting tool, removing material from the work piece. XYZ stages in these machines are equipped with means of displaying accurate coordinates of the stage's position.
A related prior art device for use in above described machines is the centering microscope. The centering microscope has been referred to as a well known prior art device in 1939 (U.S. Pat. No. 2,146,906 by Moller). The centering microscope consists of a microscope objective, a deflecting prism or mirror, and an eyepiece. The centering microscope also possesses an adapter that enables it to be mounted in spindles of machines. By viewing through the eyepiece, the operator sees a magnified image of the work piece, superimposed with crosshairs. When used in a host machine, crosshairs of a properly aligned centering microscope mark the axis of rotation of the spindle of the host machine (machine axis). A centering microscope is generally used to set up a work piece in a host machine by positioning the machine axis of the machine in a specific position on the work piece. In principle, using the centering microscope one can line up a particular feature of the part held in the host machine, note the position of the XYZ stage, line up another feature of the part on the crosshairs and note coordinates of the XYZ stage for that position, etc., and thus use the centering microscope to dimensionally inspect the part in the host machine. But the centering microscope requires operators to bring their heads down and look through the eyepiece. This position is incompatible with operating the host machine. One could use a standard eye piece adapter for a video camera. That would alleviate the problem of simultaneously viewing the part and operating the machine. But it would introduce into the setup a number of wires, for power, for video signal, for illumination, making it highly cumbersome to set up and highly inconvenient to use. Raiha in U.S. Pat. No. 4,438,567 (1984) describes a variation of the centering microscope that works in reverse by projecting a spot from an internal light source onto the work piece. The spot marks the machine axis and thus could be used in the same way as a centering microscope. The benefit would be that the user does not have to view through the eyepiece. The downside is that the accuracy of the positioning is limited to what an unaided eye can see.
A variety of stand-alone optical inspection equipment exists in the marketplace. These devices derive from layout machines such as the one described by Hawkes in U.S. Pat. No. 1,370,645 (1921). A part is placed on a tray and a pointed probe is moved around on a precision XYZ stage. The point of interest on the part is touched with a point on the probe and the coordinates of the position are read out from the stage. In one line of development the pointed mechanical probe has later on been replaced with a video camera equipped with the equivalent of a microscope objective. A digital readout of the position of the stage along all three axes is usually provided. In some versions of these machines the piece to be inspected is placed onto a XYZ translation stage while the camera is stationary. A particular feature of the part is aligned with the crosshairs integrated into the display device (such as a computer monitor) that displays a magnified image of a selected portion of the inspected part. By noting the numerical position along the three axes of the XYZ stage, as displayed on the digital readout, one can measure geometrical features of the inspected part with high precision. This stand-alone device is referred to as a Video Coordinate Measuring Machine (CMM). It is expensive and requires adequate floor space. Not every machine shop can afford it or has adequate space for it. Also, to inspect a part while in process of being machined, the part has to be taken out of the setup and brought to the CMM for inspection.
Another type of a CMM device uses an electronic touch probe instead of a video camera. The operation of these devices is conceptually similar to the video CMMs. The difference is that a part feature has to be touched with a probe rather than imaged onto crosshairs. Once the probe touches the part, the machine records the XYZ coordinates of the probe and converts them into the coordinates of the touched feature of the part. These devices, called touch probing CMMs, are also expensive and require a clean environment. Touch probes exist that can be operated in touch probe enabled milling machines (typically high end CNC milling machines). These probes can be used to inspect either in-process or finished parts. Touch probing is a useful and convenient method of inspection, but it is limited to those features that can be touched. Tiny holes, shallow steps etc. are examples of features that can not be touched.
CMMs described in this section exemplify the need for and the usefulness of dimensionally inspecting parts. These devices are typically used in QC departments away from the shop floor. Touch probes for use in touch probe enabled machines clearly indicate the need to bring inspection technology onto the production shop floor and into the production machines. Touch probe enabled machines represent a small fraction of machine tools currently in use.